A multiple-input-multiple-output (MIMO) wireless communications system comprises at least one base transceiver station (BTS) with multiple antennas and multiple mobile stations (MS), of which at least one has multiple antennas. The utilization of a beamforming technique can enhance the performance of a MIMO wireless communications system.
An L-by-N beamformed MIMO channel can be created between a BTS with M antennas and an MS with N antennas, where L≦min(M, N). The BTS computes a set of L beamforming weighting vectors by using the channel information obtained from the signals sent by the MS and the feedback on the beamformed channels from the MS. The quality of the beamforming weighting vectors is crucial to the performance of the beamformed channels.
There are a number of methods available for a BTS to compute beamforming weighting vectors by utilizing signals transmitted from antennas on the MS. One such method is to acquire the primary eigenvector of a covariance eigenvalue problem that describes the communications channel. Using this method, signals sent from a target antenna are regarded as desired signals while those sent from non-target antennas are regarded as interference signals.
Conventional methods for creating a beamformed MIMO channel assume that the channel characteristics of all signal paths are deemed the same. As a result, the transmitting signals are directly sent via the corresponding signal paths, and the transmitting power allocated to all signal paths in the beamformed MIMO channel is set to the same value. However, in reality, the channel characteristics of all signal paths of the beamformed MIMO channel are not the same. Hence, equal transmitting power among all signal paths in a beamformed MIMO channel does not ensure that they all achieve the optimal performance.
In order to achieve optimal performance, the data streams transmitted via the corresponding signal paths in a beamformed MIMO channel are adjusted according to the modulation and coding rate, and the transmitting power is adapted to the channel characteristic of the signal paths.